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Selective chemical reduction of carbon dioxide to formate using microwave irradiation
Journal of CO2 Utilization 7 (2014) 19–22
Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Selective chemical reduction of carbon dioxide to formate using microwave irradiation Indra Neel Pulidindi a, Baruchi B. Kimchi a, Aharon Gedanken a,b,* a b
Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel National Cheng Kung University, Department of Materials Science and Engineering, Tainan 70101, Taiwan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 13 May 2013 Received in revised form 22 May 2014 Accepted 5 June 2014 Available online
Selective chemical reduction of carbon dioxide to formate anion under microwave irradiation is reported. No formaldehyde or methanol was generated as by products of the CO2 reduction making the process highly selective. (NH4)2CO3 or dry ice was employed as CO2 source. NaBH4 is used as reducing agent. Aqueous solutions of (NH4)2CO3 or dry ice and NaBH4 were subjected to microwave irradiation for a short duration of 5 min. The formation of formate is unequivocally established from the 1H, 13C and DEPT-135 NMR spectroscopic studies. FT-IR and UV–vis have been employed as auxiliary tools to observe the presence of formate ions. The obtained formate has the potential to be used as fuel in fuel cells or in turn can be converted to alkanes or can be used as a precursor for the production of hydrogen. Thus the current process is highly significant for the energy sector and to the chemical industry as well. ß 2014 Elsevier Ltd. All rights reserved.
Keywords: CO2 reduction Solid CO2 Microwave irradiation HCOONa NaBH4 (NH4)2CO3
1. Introduction Conversion of CO2 to formate (yield, 75 wt.%) under microwave irradiation has been the central aspect of this paper. Chemical reduction of carbon dioxide (CO2) to fuels and chemicals is one of the dream reactions of chemists. Transformation of CO2 is significant from energy (production of fuels like methanol, formate and hydrocarbons) and environmental (climate change due to excessive accumulation of CO2) view points. Chemical reduction of CO2 is a difficult problem owing to the fact that CO2 is a very stable molecule (standard free energy of formation, DG8 = 394.3 kJ/mol). Chemical reduction methods operating under modest reaction conditions to convert carbon dioxide to formate, selectively are being actively investigated [1– 5]. Radiochemical, photochemical, electrochemical and biophotoelectrochemical methods are known to yield formic acid from carbon dioxide [6–8]. But none of the afore mentioned approaches could be employed industrially. Shinjiro Matsuoka et al., have reported the photochemical reduction of CO2 to formate (quantum yield, 0.072) using p-terphenyl as the catalyst. In addition to formate, CO and H2 were also observed as by products [9]. Sato et al., have developed a
* Corresponding author at: Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel. Tel.: +972 3 5318315; fax: +972 7384053. E-mail address: [email protected] (A. Gedanken). http://dx.doi.org/10.1016/j.jcou.2014.06.002 2212-9820/ß 2014 Elsevier Ltd. All rights reserved.
semiconductor (p-type InP)/complex hybrid (Ru complex polymer) – TiO2, photo catalyst to photoelectrochemically reduce CO2 to formate (HCOO) with a selectivity of >70% and a conversion efficiency of 0.03–0.04%. Formate (m/z = 45) was identified as the main reduction product of CO2 with ion-chromatography time-of-flight mass spectroscopy (IC-TOFMS) [10]. Guoying Zhao and Ferenc Joo have reported a rhodium-phosphine complex [RhCl(mtppms)3] based hydrogenation catalyst for the reduction of CO2 to formic acid (0.13 M) in the presence of sodium formate solutions [11]. Zheng Shen et al., have reported glycerol to be an efficient reducing agent for the conversion of CO2 (either dry ice or NaHCO3) to formate in 90% yield under a hydrothermal method [12]. Formic acid is currently gaining increased attention as a compact hydrogen energy source and is a promising hydrogen storage medium as depicted below [13–15]:
HCOOH $ CO2 þ H2 Formic acid was found to be better fuel compared to methanol for fuel cell applications. The electrode showed best tolerance to the surface poisoning when formic acid is used as a fuel compared to methanol [16,17]. Conventionally formic acid is used as reducing agent, coagulating agent, and also as a chemical precursor for the production of several alcohols, acids and esters. The formation of formic acid/formate
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anion in the hydrogenation reaction of CO2 was first reported by Farlow and Adkins in 1935 with Raney Nickel as a catalyst. Subsequently, several heterogeneous and homogeneous catalysts were developed for accelerating the process [18–26]. The reaction between NaOH and CO under pressure and at 210 8C was industrially employed for the production of HCOOH via HCOONa for several years [27]. Currently, a two stage process involving carbonylation of methanol followed by the hydrolysis of methyl formate to formic acid with the net reaction being addition of H2O to CO is widely exploited [28,29]. Herein we report a new and fast chemical reduction method for the selective conversion of CO2 to formate under microwave irradiation. NaBH4 is used as reducing agent. Either (NH4)2CO3 or dry ice in aqueous medium was used as CO2 source. Use of (NH4)2CO3 as CO2 precursor is particularly advantageous owing to the fact that the addition of (NH3, aq.) was found improve the enthalpy (from 31.2 kJ/mol to 84.3 kJ/mol) of the CO2 hydrogenation reaction [27]. In the current process, NH3 is generated in situ in addition to CO2 during the decomposition of (NH4)2CO3 and thus eliminating the necessity for the addition of NH3 ex situ. Moreover, water is the common industrial solvent for the recovery of CO2 from flue gases [28]. In addition, reduction of CO2 to liquid fuels like formate rather than gases is desirable owing to the advantages associated with transportation [13]. Thus the current methodology of carrying out the hydrogenation of CO2 reaction in aqueous solution in the presence of NH3 (aq.) is particularly attractive for industrial adoption. 2. Experimental 2.1. Materials and methods (NH4)2CO3 and NaBH4 were purchased from Sigma–Aldrich and used as received. THF (99.9%) was procured from Carlo Erba Reagents Ltd. The CO2 reduction reaction was carried out in a domestic microwave oven (DMWO, SHARP R-390F) operating at 2.45 GHz in a batch mode under atmospheric pressure in the presence of air. The output of the domestic microwave reactor was 1100 W. The microwave oven was operated at 100% power. The microwave oven was modified so as to have provision for a distillation column passing through the MW oven (for enhanced safety of operation) and also stirring facility during the reaction [30–32]. Typical process comprise of placing in the reaction vessel (a round bottom flask, 100 mL), 0.35 g (0.00364 moles) (NH4)2CO3 (CO2 precursor) and 0.35 g (0.0092 moles) NaBH4 in 35 mL water and subsequently subjecting the contents to microwave irradiation for 5 min. In the case of dry ice, 5.0 g dry ice, 1.0 g NaBH4 and 20 mL water were placed in a round bottom flask and subjected to microwave irradiation for different periods of time (1–5 min.). The electricity consumed for the operation of the domestic microwave oven for 5 min was evaluated and was found to be 107 Wh [30]. The reaction temperature attained (90 8C) as a result of microwave irradiation is evaluated using a pyrometer (Fluke, 65 Infrared thermometer). The experimental set up for the microwave assisted CO2 reduction process is depicted in Fig. S1. For comparison, the CO2 reduction reaction was also carried out under hydrothermal conditions (at 90 8C) in a cylindrical stainless steel reactor with polytetrafluoroethylene lined inside. Typical batch of the hydrothermal reaction comprised of taking 0.35 g (0.00364 moles) (NH4)2CO3 (CO2 precursor) and 0.35 g (0.0092 moles) NaBH4 in 35 mL water in the reactor and placing the same in an air oven at the preset temperature. The reaction is quenched after the set reaction time by placing the same in a cold water bath. The reaction products were analyzed by 1H, 13C, DEPT-135 NMR, FT-IR and UV– vis spectroscopic studies. Quantification of the reaction product, HCOONa, was done with the use of 1H NMR spectroscopy with D2O
as solvent and using THF as the internal standard (Figs. S2 and S3). Typical sample for analysis comprise of 400 mL of the analyte and 100 mL of the solvent and 50 mL of internal standard (THF). Spectral analysis was carried out at room temperature. Owing to the ease involved in the sample preparation, NMR analysis was preferred over the regular mass spectrometric analysis [33,34]. UV–vis spectra were recorded on Cary 100 Scan Varian. FT-IR spectra were recorded on Bruker Tensor 27. 13C and 1H NMR spectra were recorded on Bruker Avance DPX 300. 3. Results and discussion The reaction product obtained upon microwave irradiation of (NH4)2CO3 (0.35 g in 35 mL H2O) in the presence of NaBH4 (0.35 g) for 5 min. is analyzed by 1H and 13C NMR spectroscopy. A singlet resonance at 8.3 ppm in the 1H NMR (Fig. 1a) and a resonance at 171 ppm in 13C NMR (Fig. 1b) establish the presence of formate anion and rule out the presence of formaldehyde or methanol or even carbonic acid. The yield of HCOONa was found to be 0.186 g (Fig. S4(a)). Theoretically, 1.0 g (NH4)2CO3, upon decomposition, is supposed to yield 0.46 g CO2 and correspondingly 0.71 g HCOONa with appropriate amount of NaBH4 (0.395 g). Calculations for the evaluation of the yield of formate were shown in the supporting material. A wt.% yield of 75 wt.% is obtained from a batch reaction with 0.35 g (NH4)2CO3 with an irradiation time of 5 min. This value of yield of HCOONa is also substantiated by the fact that in the 13C NMR spectrum of the reaction product (Fig. 1(b)), in addition to the intense signal at 171.1 ppm corresponding to the HCOONa, an additional signal of relatively lower intensity is observed at 163.1 ppm. Moreover, in the DEPT-135 NMR spectrum of the corresponding sample (Fig. 3(b)), the peak at 163.1 is absent and only the peak at 171.1 ppm is present. This points to the fact that the peak at 163.1 ppm corresponds to the unreacted traces of (NH4)2CO3 as carbonate samples do not show any signal in DEPT NMR. More over, the 13C NMR spectrum of the aqueous solution of (NH4)2CO3 (commercial sample) showed sharp singlet signal at 162.3 ppm (Fig. S5) which indeed confirm the presence of traces of unreacted (NH4)2CO3. In addition, the positive signal in the DEPT 135 NMR at 171.1 ppm confirm that the reaction product is indeed HCOONa.
Fig. 1. (a) 1H and (b) 13C NMR spectra of reaction product from the chemical reduction of CO2 (from (NH4)2CO3).
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Fig. 2. (a) 1H and (b) 13C NMR spectra of reaction product from the chemical reduction of CO2 (from dry ice).
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of the hydrogenation of CO2 from dry ice is indeed due to the formation of Na2CO3. In addition, so as to disprove the attribution of this peak at 167.4 ppm to NaHCO3, the 13C NMR spectrum of authentic sample of NaHCO3 is also recorded which showed a sharp intense peak at 160.8 ppm but not at 167.4 (Fig. S7). This indicates that Na2CO3 but not NaHCO3 is present as impurity in the reaction of hydrogenation of dry ice to yield HCOONa. The yield of HCOONa obtained from the hydrogenation of dry ice was found to be 0.25 g (Fig. S4(b)). The % yield is 5%. The lower yield of HCOONa from dry ice relative to (NH4)2CO3 is due to the fact that the amount of NaBH4 taken along with dry ice is lower (dry ice: NaBH4 = 5:1, wt./wt. ratio). Moreover, the absence of NH3 (aq.) in the case of dry ice also had a negative impact on the dissolution as well as the hydrogenation of CO2. Thus, either dry ice or in situ generated CO2 from (NH4)2CO3 could indeed be reduced by NaBH4 upon exposure to microwave irradiation yielding formate anion. Thus the current research finding is a breakthrough in the chemistry of CO2. DEPT-135 NMR is another authentic tool to unequivocally establish the presence of formate anion. In the DEPT-135 NMR spectrum the positive signals indicate methine (BCH) carbons, negative signals indicate methylene carbons (5 5CH) and no signals appear for quaternary carbons (C). The DEPT-135 NMR spectra of commercial HCOONa (aq.) (Fig. 3(a)), the reaction product
To evaluate the effect of microwave irradiation time on the yield of formate form CO2 reduction, the reaction was carried out at 1 and 3 min and the yields of formate are 39 and 60 wt.% respectively. Thus the maximum yield (75 wt.%) could be obtained by irradiating the CO2 precursor with the reducing agent for 5 min. For comparison, the reaction was also studied under hydrothermal conditions at identical reaction conditions and the yield of formate is only 49 wt.% even after 3 h of reaction. Thus microwave irradiation offers a fast way for the reduction of CO2 to formate. Similar results were obtained when 5.0 g dry ice, 1.0 g NaBH4 and 20 mL water were placed in a round bottom flask and subjected to microwave irradiation for 5 min. The singlet signal at 8.3 ppm in 1H NMR (Fig. 2(a)) and the signal at 171.1 ppm in 13C NMR confirm the formation of HCOONa from dry ice. In addition to the sharp signal at 171.1 ppm in the 13C NMR spectrum, an additional signal of lower intensity is observed at 167.4 ppm (Fig. 2(b)). The afore mentioned signal at 167.4 ppm is absent in the DEPT-135 NMR spectrum (Fig. 3(c)) indicating that the origin of the peak is due to the presence of the trace of carbonate that is present in the reaction product. So as to verify if the peak at 167.4 ppm is indeed due to the formation of Na2CO3 that is formed in situ during the microwave irradiation of dry ice and NaBH4, the 13C NMR spectrum of Na2CO3 in D2O solvent is recorded and an intense signal at 168.1 ppm is observed (Fig. S6). This confirms that the additional peak at 167.4 ppm in the reaction
Fig. 3. DEPT-135 NMR spectra of (a) commercial HCOONa (aqueous solution), (b) reaction product when (NH4)2CO3 is employed as CO2 precursor and (c) reaction product when dry ice is employed as CO2 precursor.
Fig. 4. (A) FT-IR spectrum (liquid, RT) of the reaction product from the chemical reduction of (NH4)2CO3 in the presence of NaBH4 for 5 min and (B) UV–vis spectra (liquid, RT) of (a) the reaction product from the chemical reduction of (NH4)2CO3 in the presence of NaBH4 for 5 min and (b) the aqueous solution of HCOONa (authentic sample).
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obtained when (NH4)2CO3 is employed as CO2 precursor (Fig. 3(b)), the reaction product obtained when dry ice (in water) is used as CO2 source (Fig. 3(c)), are shown in Fig. 3. In all the three cases, the presence of a positive peak in the DEPT-135 spectra at 171 ppm is observed which is characteristic of the formate anion. Thus, irrespective of the CO2 source (either (NH4)2CO3 or dry ice), the chemical reduction methodology currently developed yields the formate anion selectively from CO2. The reaction product obtained from the chemical reduction of CO2 generated in situ from (NH4)2CO3 in the presence of NaBH4 upon microwave irradiation is characterized by FT-IR (Fig. 4A). The bands at 1394 and 1640 cm1 correspond to the C–O and C5 5O bond stretching modes and are an indication of the presence of sodium formate in the analyte. The spectral features are similar to the FT-IR spectrum of HCOONa [35]. The UV–vis spectrum of the reaction product of the above mentioned precursor is depicted in Fig. 4(B). For comparison the UV–vis spectrum of the commercial sample of HCOONa (aqueous solution) is also shown in Fig. 4B(b). An absorption maxima at 215 nm is observed in both the afore mentioned cases indicating that the chemical reduction product of CO2 consists of formate anions and also the typical absorption around 215 nm originating from the reaction product (formate) can be used as an index for identifying the presence of formate anion. 4. Conclusion CO2, either generated in situ by the decomposition of (NH4)2CO3 or from dry ice, in aqueous medium is reduced selectively to formate using NaBH4 as reducing agent under microwave irradiation for 5 min. The chemical reduction process involving the conversion of CO2 to formate in aqueous medium is fast owing to the use of microwave irradiation. The exclusive formation of HCOONa as the CO2 chemical reduction product is established authentically through 1H, 13C, DEPT-135 NMR, FT-IR and UV–vis spectroscopic studies. NaBH4 is an efficient reducing agent for the reduction of CO2 in aqueous medium under microwave irradiation conditions. The mechanism of this reduction reaction is currently being investigated. Thus a new process is developed for the chemical reduction of the thermodynamically stable CO2 molecule to be converted to a valuable fuel, formate, which can be used as such in fuel cells as a substitute to methanol or can be employed as a reservoir for hydrogen fuel or can be used as an industrial chemical. Based on the current methodology, the industrial effluent, CO2 gas could be selectively converted to HCOONa which is an environmentally benign fuel for transportation application making the process of immense commercial significance and beneficial to the electrical companies generating electricity by the combustion of coal and emanating CO2 is surplus volumes. In addition, the process is useful to most of the industries releasing CO2 into the environment. The fuel cell technology is also upcoming which needs the fuels such as HCOONa for rapid commercialization. Further studies on the economic feasibility of the process and the direct conversion of CO2 are underway. Acknowledgements Gedanken thanks the (MOST) for the research grant 3-9802 and the Israel Science Foundation (ISF) for supporting the research via a grant 12/586. The authors are thankful to Dr. Hugo Gottlieb, BarIlan University, Israel, for the fruitful discussions.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:10.1016/j.jcou.2014.06.002. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]
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Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Selective chemical reduction of carbon dioxide to formate using microwave irradiation Indra Neel Pulidindi a, Baruchi B. Kimchi a, Aharon Gedanken a,b,* a b
Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel National Cheng Kung University, Department of Materials Science and Engineering, Tainan 70101, Taiwan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 13 May 2013 Received in revised form 22 May 2014 Accepted 5 June 2014 Available online
Selective chemical reduction of carbon dioxide to formate anion under microwave irradiation is reported. No formaldehyde or methanol was generated as by products of the CO2 reduction making the process highly selective. (NH4)2CO3 or dry ice was employed as CO2 source. NaBH4 is used as reducing agent. Aqueous solutions of (NH4)2CO3 or dry ice and NaBH4 were subjected to microwave irradiation for a short duration of 5 min. The formation of formate is unequivocally established from the 1H, 13C and DEPT-135 NMR spectroscopic studies. FT-IR and UV–vis have been employed as auxiliary tools to observe the presence of formate ions. The obtained formate has the potential to be used as fuel in fuel cells or in turn can be converted to alkanes or can be used as a precursor for the production of hydrogen. Thus the current process is highly significant for the energy sector and to the chemical industry as well. ß 2014 Elsevier Ltd. All rights reserved.
Keywords: CO2 reduction Solid CO2 Microwave irradiation HCOONa NaBH4 (NH4)2CO3
1. Introduction Conversion of CO2 to formate (yield, 75 wt.%) under microwave irradiation has been the central aspect of this paper. Chemical reduction of carbon dioxide (CO2) to fuels and chemicals is one of the dream reactions of chemists. Transformation of CO2 is significant from energy (production of fuels like methanol, formate and hydrocarbons) and environmental (climate change due to excessive accumulation of CO2) view points. Chemical reduction of CO2 is a difficult problem owing to the fact that CO2 is a very stable molecule (standard free energy of formation, DG8 = 394.3 kJ/mol). Chemical reduction methods operating under modest reaction conditions to convert carbon dioxide to formate, selectively are being actively investigated [1– 5]. Radiochemical, photochemical, electrochemical and biophotoelectrochemical methods are known to yield formic acid from carbon dioxide [6–8]. But none of the afore mentioned approaches could be employed industrially. Shinjiro Matsuoka et al., have reported the photochemical reduction of CO2 to formate (quantum yield, 0.072) using p-terphenyl as the catalyst. In addition to formate, CO and H2 were also observed as by products [9]. Sato et al., have developed a
* Corresponding author at: Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel. Tel.: +972 3 5318315; fax: +972 7384053. E-mail address: [email protected] (A. Gedanken). http://dx.doi.org/10.1016/j.jcou.2014.06.002 2212-9820/ß 2014 Elsevier Ltd. All rights reserved.
semiconductor (p-type InP)/complex hybrid (Ru complex polymer) – TiO2, photo catalyst to photoelectrochemically reduce CO2 to formate (HCOO) with a selectivity of >70% and a conversion efficiency of 0.03–0.04%. Formate (m/z = 45) was identified as the main reduction product of CO2 with ion-chromatography time-of-flight mass spectroscopy (IC-TOFMS) [10]. Guoying Zhao and Ferenc Joo have reported a rhodium-phosphine complex [RhCl(mtppms)3] based hydrogenation catalyst for the reduction of CO2 to formic acid (0.13 M) in the presence of sodium formate solutions [11]. Zheng Shen et al., have reported glycerol to be an efficient reducing agent for the conversion of CO2 (either dry ice or NaHCO3) to formate in 90% yield under a hydrothermal method [12]. Formic acid is currently gaining increased attention as a compact hydrogen energy source and is a promising hydrogen storage medium as depicted below [13–15]:
HCOOH $ CO2 þ H2 Formic acid was found to be better fuel compared to methanol for fuel cell applications. The electrode showed best tolerance to the surface poisoning when formic acid is used as a fuel compared to methanol [16,17]. Conventionally formic acid is used as reducing agent, coagulating agent, and also as a chemical precursor for the production of several alcohols, acids and esters. The formation of formic acid/formate
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anion in the hydrogenation reaction of CO2 was first reported by Farlow and Adkins in 1935 with Raney Nickel as a catalyst. Subsequently, several heterogeneous and homogeneous catalysts were developed for accelerating the process [18–26]. The reaction between NaOH and CO under pressure and at 210 8C was industrially employed for the production of HCOOH via HCOONa for several years [27]. Currently, a two stage process involving carbonylation of methanol followed by the hydrolysis of methyl formate to formic acid with the net reaction being addition of H2O to CO is widely exploited [28,29]. Herein we report a new and fast chemical reduction method for the selective conversion of CO2 to formate under microwave irradiation. NaBH4 is used as reducing agent. Either (NH4)2CO3 or dry ice in aqueous medium was used as CO2 source. Use of (NH4)2CO3 as CO2 precursor is particularly advantageous owing to the fact that the addition of (NH3, aq.) was found improve the enthalpy (from 31.2 kJ/mol to 84.3 kJ/mol) of the CO2 hydrogenation reaction [27]. In the current process, NH3 is generated in situ in addition to CO2 during the decomposition of (NH4)2CO3 and thus eliminating the necessity for the addition of NH3 ex situ. Moreover, water is the common industrial solvent for the recovery of CO2 from flue gases [28]. In addition, reduction of CO2 to liquid fuels like formate rather than gases is desirable owing to the advantages associated with transportation [13]. Thus the current methodology of carrying out the hydrogenation of CO2 reaction in aqueous solution in the presence of NH3 (aq.) is particularly attractive for industrial adoption. 2. Experimental 2.1. Materials and methods (NH4)2CO3 and NaBH4 were purchased from Sigma–Aldrich and used as received. THF (99.9%) was procured from Carlo Erba Reagents Ltd. The CO2 reduction reaction was carried out in a domestic microwave oven (DMWO, SHARP R-390F) operating at 2.45 GHz in a batch mode under atmospheric pressure in the presence of air. The output of the domestic microwave reactor was 1100 W. The microwave oven was operated at 100% power. The microwave oven was modified so as to have provision for a distillation column passing through the MW oven (for enhanced safety of operation) and also stirring facility during the reaction [30–32]. Typical process comprise of placing in the reaction vessel (a round bottom flask, 100 mL), 0.35 g (0.00364 moles) (NH4)2CO3 (CO2 precursor) and 0.35 g (0.0092 moles) NaBH4 in 35 mL water and subsequently subjecting the contents to microwave irradiation for 5 min. In the case of dry ice, 5.0 g dry ice, 1.0 g NaBH4 and 20 mL water were placed in a round bottom flask and subjected to microwave irradiation for different periods of time (1–5 min.). The electricity consumed for the operation of the domestic microwave oven for 5 min was evaluated and was found to be 107 Wh [30]. The reaction temperature attained (90 8C) as a result of microwave irradiation is evaluated using a pyrometer (Fluke, 65 Infrared thermometer). The experimental set up for the microwave assisted CO2 reduction process is depicted in Fig. S1. For comparison, the CO2 reduction reaction was also carried out under hydrothermal conditions (at 90 8C) in a cylindrical stainless steel reactor with polytetrafluoroethylene lined inside. Typical batch of the hydrothermal reaction comprised of taking 0.35 g (0.00364 moles) (NH4)2CO3 (CO2 precursor) and 0.35 g (0.0092 moles) NaBH4 in 35 mL water in the reactor and placing the same in an air oven at the preset temperature. The reaction is quenched after the set reaction time by placing the same in a cold water bath. The reaction products were analyzed by 1H, 13C, DEPT-135 NMR, FT-IR and UV– vis spectroscopic studies. Quantification of the reaction product, HCOONa, was done with the use of 1H NMR spectroscopy with D2O
as solvent and using THF as the internal standard (Figs. S2 and S3). Typical sample for analysis comprise of 400 mL of the analyte and 100 mL of the solvent and 50 mL of internal standard (THF). Spectral analysis was carried out at room temperature. Owing to the ease involved in the sample preparation, NMR analysis was preferred over the regular mass spectrometric analysis [33,34]. UV–vis spectra were recorded on Cary 100 Scan Varian. FT-IR spectra were recorded on Bruker Tensor 27. 13C and 1H NMR spectra were recorded on Bruker Avance DPX 300. 3. Results and discussion The reaction product obtained upon microwave irradiation of (NH4)2CO3 (0.35 g in 35 mL H2O) in the presence of NaBH4 (0.35 g) for 5 min. is analyzed by 1H and 13C NMR spectroscopy. A singlet resonance at 8.3 ppm in the 1H NMR (Fig. 1a) and a resonance at 171 ppm in 13C NMR (Fig. 1b) establish the presence of formate anion and rule out the presence of formaldehyde or methanol or even carbonic acid. The yield of HCOONa was found to be 0.186 g (Fig. S4(a)). Theoretically, 1.0 g (NH4)2CO3, upon decomposition, is supposed to yield 0.46 g CO2 and correspondingly 0.71 g HCOONa with appropriate amount of NaBH4 (0.395 g). Calculations for the evaluation of the yield of formate were shown in the supporting material. A wt.% yield of 75 wt.% is obtained from a batch reaction with 0.35 g (NH4)2CO3 with an irradiation time of 5 min. This value of yield of HCOONa is also substantiated by the fact that in the 13C NMR spectrum of the reaction product (Fig. 1(b)), in addition to the intense signal at 171.1 ppm corresponding to the HCOONa, an additional signal of relatively lower intensity is observed at 163.1 ppm. Moreover, in the DEPT-135 NMR spectrum of the corresponding sample (Fig. 3(b)), the peak at 163.1 is absent and only the peak at 171.1 ppm is present. This points to the fact that the peak at 163.1 ppm corresponds to the unreacted traces of (NH4)2CO3 as carbonate samples do not show any signal in DEPT NMR. More over, the 13C NMR spectrum of the aqueous solution of (NH4)2CO3 (commercial sample) showed sharp singlet signal at 162.3 ppm (Fig. S5) which indeed confirm the presence of traces of unreacted (NH4)2CO3. In addition, the positive signal in the DEPT 135 NMR at 171.1 ppm confirm that the reaction product is indeed HCOONa.
Fig. 1. (a) 1H and (b) 13C NMR spectra of reaction product from the chemical reduction of CO2 (from (NH4)2CO3).
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Fig. 2. (a) 1H and (b) 13C NMR spectra of reaction product from the chemical reduction of CO2 (from dry ice).
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of the hydrogenation of CO2 from dry ice is indeed due to the formation of Na2CO3. In addition, so as to disprove the attribution of this peak at 167.4 ppm to NaHCO3, the 13C NMR spectrum of authentic sample of NaHCO3 is also recorded which showed a sharp intense peak at 160.8 ppm but not at 167.4 (Fig. S7). This indicates that Na2CO3 but not NaHCO3 is present as impurity in the reaction of hydrogenation of dry ice to yield HCOONa. The yield of HCOONa obtained from the hydrogenation of dry ice was found to be 0.25 g (Fig. S4(b)). The % yield is 5%. The lower yield of HCOONa from dry ice relative to (NH4)2CO3 is due to the fact that the amount of NaBH4 taken along with dry ice is lower (dry ice: NaBH4 = 5:1, wt./wt. ratio). Moreover, the absence of NH3 (aq.) in the case of dry ice also had a negative impact on the dissolution as well as the hydrogenation of CO2. Thus, either dry ice or in situ generated CO2 from (NH4)2CO3 could indeed be reduced by NaBH4 upon exposure to microwave irradiation yielding formate anion. Thus the current research finding is a breakthrough in the chemistry of CO2. DEPT-135 NMR is another authentic tool to unequivocally establish the presence of formate anion. In the DEPT-135 NMR spectrum the positive signals indicate methine (BCH) carbons, negative signals indicate methylene carbons (5 5CH) and no signals appear for quaternary carbons (C). The DEPT-135 NMR spectra of commercial HCOONa (aq.) (Fig. 3(a)), the reaction product
To evaluate the effect of microwave irradiation time on the yield of formate form CO2 reduction, the reaction was carried out at 1 and 3 min and the yields of formate are 39 and 60 wt.% respectively. Thus the maximum yield (75 wt.%) could be obtained by irradiating the CO2 precursor with the reducing agent for 5 min. For comparison, the reaction was also studied under hydrothermal conditions at identical reaction conditions and the yield of formate is only 49 wt.% even after 3 h of reaction. Thus microwave irradiation offers a fast way for the reduction of CO2 to formate. Similar results were obtained when 5.0 g dry ice, 1.0 g NaBH4 and 20 mL water were placed in a round bottom flask and subjected to microwave irradiation for 5 min. The singlet signal at 8.3 ppm in 1H NMR (Fig. 2(a)) and the signal at 171.1 ppm in 13C NMR confirm the formation of HCOONa from dry ice. In addition to the sharp signal at 171.1 ppm in the 13C NMR spectrum, an additional signal of lower intensity is observed at 167.4 ppm (Fig. 2(b)). The afore mentioned signal at 167.4 ppm is absent in the DEPT-135 NMR spectrum (Fig. 3(c)) indicating that the origin of the peak is due to the presence of the trace of carbonate that is present in the reaction product. So as to verify if the peak at 167.4 ppm is indeed due to the formation of Na2CO3 that is formed in situ during the microwave irradiation of dry ice and NaBH4, the 13C NMR spectrum of Na2CO3 in D2O solvent is recorded and an intense signal at 168.1 ppm is observed (Fig. S6). This confirms that the additional peak at 167.4 ppm in the reaction
Fig. 3. DEPT-135 NMR spectra of (a) commercial HCOONa (aqueous solution), (b) reaction product when (NH4)2CO3 is employed as CO2 precursor and (c) reaction product when dry ice is employed as CO2 precursor.
Fig. 4. (A) FT-IR spectrum (liquid, RT) of the reaction product from the chemical reduction of (NH4)2CO3 in the presence of NaBH4 for 5 min and (B) UV–vis spectra (liquid, RT) of (a) the reaction product from the chemical reduction of (NH4)2CO3 in the presence of NaBH4 for 5 min and (b) the aqueous solution of HCOONa (authentic sample).
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obtained when (NH4)2CO3 is employed as CO2 precursor (Fig. 3(b)), the reaction product obtained when dry ice (in water) is used as CO2 source (Fig. 3(c)), are shown in Fig. 3. In all the three cases, the presence of a positive peak in the DEPT-135 spectra at 171 ppm is observed which is characteristic of the formate anion. Thus, irrespective of the CO2 source (either (NH4)2CO3 or dry ice), the chemical reduction methodology currently developed yields the formate anion selectively from CO2. The reaction product obtained from the chemical reduction of CO2 generated in situ from (NH4)2CO3 in the presence of NaBH4 upon microwave irradiation is characterized by FT-IR (Fig. 4A). The bands at 1394 and 1640 cm1 correspond to the C–O and C5 5O bond stretching modes and are an indication of the presence of sodium formate in the analyte. The spectral features are similar to the FT-IR spectrum of HCOONa [35]. The UV–vis spectrum of the reaction product of the above mentioned precursor is depicted in Fig. 4(B). For comparison the UV–vis spectrum of the commercial sample of HCOONa (aqueous solution) is also shown in Fig. 4B(b). An absorption maxima at 215 nm is observed in both the afore mentioned cases indicating that the chemical reduction product of CO2 consists of formate anions and also the typical absorption around 215 nm originating from the reaction product (formate) can be used as an index for identifying the presence of formate anion. 4. Conclusion CO2, either generated in situ by the decomposition of (NH4)2CO3 or from dry ice, in aqueous medium is reduced selectively to formate using NaBH4 as reducing agent under microwave irradiation for 5 min. The chemical reduction process involving the conversion of CO2 to formate in aqueous medium is fast owing to the use of microwave irradiation. The exclusive formation of HCOONa as the CO2 chemical reduction product is established authentically through 1H, 13C, DEPT-135 NMR, FT-IR and UV–vis spectroscopic studies. NaBH4 is an efficient reducing agent for the reduction of CO2 in aqueous medium under microwave irradiation conditions. The mechanism of this reduction reaction is currently being investigated. Thus a new process is developed for the chemical reduction of the thermodynamically stable CO2 molecule to be converted to a valuable fuel, formate, which can be used as such in fuel cells as a substitute to methanol or can be employed as a reservoir for hydrogen fuel or can be used as an industrial chemical. Based on the current methodology, the industrial effluent, CO2 gas could be selectively converted to HCOONa which is an environmentally benign fuel for transportation application making the process of immense commercial significance and beneficial to the electrical companies generating electricity by the combustion of coal and emanating CO2 is surplus volumes. In addition, the process is useful to most of the industries releasing CO2 into the environment. The fuel cell technology is also upcoming which needs the fuels such as HCOONa for rapid commercialization. Further studies on the economic feasibility of the process and the direct conversion of CO2 are underway. Acknowledgements Gedanken thanks the (MOST) for the research grant 3-9802 and the Israel Science Foundation (ISF) for supporting the research via a grant 12/586. The authors are thankful to Dr. Hugo Gottlieb, BarIlan University, Israel, for the fruitful discussions.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:10.1016/j.jcou.2014.06.002. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]
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